In 2.4-GHz band referred to as “ISM (Industry Science Medical) band”, a user can use a wireless device without requiring any license, as long as the criteria defined in the Radio Act is satisfied. Therefore, in recent years, wireless devices using the 2.4-GHz band such as wireless LAN (Local Area Network) (the IEEE (the Institute of Electrical and Electronics Engineers) 802.11b/g/h), Bluetooth®, and cordless phones have been actively developed.
In a wireless device compliant to the IEEE 802.11b/g/h (hereinafter, “WLAN (Wireless LAN) device”), direct sequence spread spectrum (DSSS) and OFDM (Orthogonal Frequency Division Multiplexing) techniques have been introduced, taking noise immunity into consideration. In this regard, communication is performed by fixedly using one of previously-defined 14 channels in the 2.4-GHz ISM band (one channel has an occupied frequency bandwidth corresponding to about 20 channels of a Bluetooth® device).
Furthermore, taking interoperability with another network or another system into consideration, wireless access CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) has been mainly introduced, in which when respective wireless terminals perform carrier sense of a wireless channel prior to transmission of a wireless packet, and when having confirmed that a channel is being used (channel busy), the wireless terminal withholds transmission of the wireless packet, and transmits the wireless packet after the time lapse of a channel unused time (a channel idle time) and a back-off time predetermined for each frame type.
In a Bluetooth®-compatible wireless communication device (hereinafter, “BT device”), the noise immunity is similarly taken into consideration, and frequency hopping spread spectrum (FHSS) technique has been introduced. Further, a frequency hopping system in which among 79 frequency channels having predetermined 1-MHz width (hereinafter “FH channels”) in a frequency band from 2.40 to 2.48 gigahertz, one FH channel is selected and switched with the passage of time to perform wireless communication has been adopted. In this frequency hopping system, a FH channel is selected so as to be repeated with a certain time interval (for example, 625 microseconds) based on a predetermined pseudo random algorithm, and one packet data is allocated to one FH channel to perform communication.
Therefore, if a BT device and a WLAN device that perform communication by using the same 2.4-GHz band are present in their respective communication areas, radio waves transmitted from respective devices interfere with each other, to interrupt respective communications. As a method of avoiding the radio wave interference, a technique referred to as “Adaptive Frequency Hopping: AFH” has been disclosed in Patent Literatures 1 and 2 mentioned below. This technique prevents interference from other systems such as the WLAN device by measuring a bit error rate (BER) or a packet error rate (PER) during transmission or measuring received signal strength in a slot in which communication is not being performed between BT devices to observe the quality of the FH channel on the BT device side (interference susceptibility from other systems such as the WLAN device), and performing frequency hopping, avoiding an FH channel in which a radio wave interfering its own communication is determined to be present.
For example, in the AFH technique, it is assumed that an FH channel determined such that there is no radio wave interrupting its own communication is a “good channel: Good”, an FH channel determined such that there is a radio wave interrupting its own communication is a “bad channel: Bad”, and an FH channel that cannot be determined is an “unknown channel: Unknown”, and these channels are held in AFH_channel_map and AFH_channel_classification. The BT device then performs communication by using an FH channel as good as possible, by sharing the FH channel between a master station and a slave station by using LMP_set_AFH, which is a Link Manager Protocol, or the like.
However, because the conventional AFH technique mentioned above is performed, taking only the quality on the BT device side into consideration, considerable interruption may be caused in the communication between the WLAN devices by using an FH channel determined that there is no problem in the communication between the BT devices. For example, if a WLAN device falsely detects a signal outside a band transmitted from a BT device as a signal having a signal strength exceeding a predetermined threshold by performing carrier sense, the WLAN device waits before transmission even in a channel actually usable, thereby causing an unnecessary decrease in the communication throughput. Particularly, in a composite wireless device having both functions of the WLAN device and the BT device, the above problem becomes serious.
In the conventional AFH technique, further, when the use bandwidth of the WLAN device increases, the number of FH channels determined to be usable without any interruption by the BT device (good channels) becomes insufficient. Therefore, to satisfy the standard of the Bluetooth®, some of FH channels need to be used as usable channels, among the FH channels that do not satisfy the predetermined quality (bad channels) for the BT device due to interference with the WLAN device.
A method of avoiding deterioration of interference robustness of the BT device due to insufficient good channels is disclosed in paragraph 7 of Non Patent Literature 1 mentioned below and Patent Literature 3 mentioned below.
Furthermore, as a technique for avoiding interference between the BT device and the WLAN device described above, a method of controlling a transmission and reception timing of each BT device and WLAN device in a composite wireless device, in which both the WLAN device and the BT device are mounted on one wireless terminal, by mediating communication states between the BT device and the WLAN device, is disclosed in Non Patent Literature 1 mentioned below.
Further, Patent Literature 4 mentioned below discloses such a method that in a composite wireless device mounted with both the WLAN device and the BT device, the WLAN device itself ascertains not only an operating frequency of its own device but also channels used by other peripheral WLAN devices, and notifies the information to the BT device, so that the BT device selects an FH channel. Patent Literature 4 mentioned below further discloses a method of selecting an FH channel that can be used by the BT device based on communication timing information indicating the communication state of the WLAN device, such as an FH channel that cannot be used at a transmission timing of the WLAN device (an unusable channel at the time of transmission) according to the transmission and reception timing of the WLAN device, an FH channel that cannot be used at a carrier sense timing of the WLAN device (a false-detection risk channel), and an FH channel that cannot be used at a reception timing of the WLAN device (an unusable channel at the time of reception). Patent Literature 4 mentioned below also discloses such a method that FH channels are selected as quasi-usable FH channels from FH channels included in each WLAN channel predicted to be being used by other WLAN devices, excluding the false-detection risk channel corresponding to a WLAN channel to be used by, for example, its own WLAN device, in order from an FH channel farthest from a center frequency of the WLAN channel by alternately selecting an FH channel having a high frequency and an FH channel having a low frequency until the number of insufficient channels is reached, thereby compensating shortage of the number of good channels.
Patent Literature 5 mentioned below discloses a method in which a first wireless communication system transmits a CTS (Clear to Send) or an RTS (Request to Send)/CTS frame to reserve a communication band for a second wireless communication system, in order to ensure the communication band for the second wireless communication system.